Generation of images or other useful signals responsive to detected millimeter waves (radiation having wavelengths in approximately the 1 cm-1 mm range, that is, of between approximately 30 and 300 GHz frequency) reflected from or emitted by objects in a field of view is desired in many applications. This is largely because millimeter waves are not as completely attenuated by moisture in the air, as are, for example, visible and infrared radiation. For example, the fact that visible light is very thoroughly attenuated by fog currently prevents aircraft from landing and taking off in fog, which obviously is highly inconvenient to travellers. Millimeter waves also penetrate other adverse environments such as smoke, clouds of chemical gas, dust and the like wherein the particles are of less than millimeter size.
A further advantage of millimeter waves, particularly with respect to microwaves, is that many of the components, notably antennas, can be made much smaller due to the shorter wavelength of the radiation. Accordingly, it would be desirable to use millimeter waves in various applications where smaller antennas are needed, for example, in aircraft applications, or in other mobile, marine or space applications.
There are several reasons why practical millimeter wave imaging systems are not now available. One is that to generate an image using a millimeter-wave sensor has been thought to require either mechanical or electronic scanning of the sensor with respect to the field of view. Mechanical scanning devices in which a sensor is physically moved through a range of azimuths, elevations or both, defining a field of view, are complex and subject to failure. Electronic scanning is also complex and at millimeter-wave frequencies requires employment of electronic phase shifting or switching techniques, which are relatively complex to implement.
More specifically, electronic scanning systems proposed to date have involved aperture-plane arrays, that is, arrays of radiation sources, which emit radiation which varies in phase from one emitter to the next. Such "phased-array" systems are described in Introduction to Radar Systems, Skolnik (1980), especially in chapter 8, pp. 278-342. Broadly, the signal transmitted by each of the sources travels outwardly in a different direction; a single detector element detects radiation reflected from any objects in the field of view, and the phase of the detected radiation is used to determine the azimuth and elevation of the reflecting object.
Such phased-array systems are complex by their nature; the complexity would only be compounded by the high frequencies inherent in millimeter-wave systems.
One conventional method of reducing the frequency of a received signal to a lower frequency for convenience in signal processing is to mix the received signal with a local oscillator signal of generally similar frequency. As is well known, this "mixing" results in sum and difference signals. For example, Skolnik, op. cit., discusses at page 82 that a diode or other non-linear element may be used in a radar system to heterodyne an echo signal with a portion of the transmitter signal resulting in a difference signal or "beat note". The difference signal is reduced in frequency, such that it can be processed using more conventional electronic circuitry and techniques. However, normally a millimeter-wave local oscillator signal must be combined with the received signal using waveguide or transmission line technology; while not infeasible, this requirement has limited millimeter-wave receivers to a single imaging element, which therefore must be scanned either mechanically or electronically as described above to generate a video image of an object to be imaged. As noted, either scanning technique introduces substantial complexity to an imaging system. Furthermore, it would be desirable to provide a system in which millimeter-wave energy is transmitted from an emitter onto the field of view to illuminate it. It is difficult at present to construct a sufficiently powerful oscillator to provide an adequately strong received signal, as present day solid state millimeter wave sources such as Gunn diode oscillators and the like are limited in their millimeter wave power output.
References are known which suggest that a focal plane array of antenna elements responsive to millimeter wave radiation can be constructed. See Gillespie et al, "Array Detectors for Millimeter Line Astronomy" Astron, Astrophys, 73, 14-18 (1979). This reference shows an array of elements for detection of millimeter wave radiation, in which a local oscillator signals is introduced from a central feed area of a primary mirror of a Cassegrain telescope. As acknowledged by Gillespie et al, this would lead to serious difficulties with uniformity of the local oscillator signal over the array. Furthermore, Gillespie et al only teaches a single element detector, that is, Gillespie et al does not teach a multiple element array in which each pixel of the image correspond to one of the detectors.
Other references show systems in which the elements of the focal plane array are intended to map to elements of the ultimate image. See, for example, Yngvesson et al "Millimeter Wave Imaging System With An Endfire Receptor Array", 10th International Conference on Infrared and Millimeter Waves (1985). Other references of comparable disclosure are also known. This document suggests a multiple element focal plane array wherein each element includes a diode connected across spaced antenna elements for rectification of received millimeter-wave energy and superheterodyne signal detection. Yngvesson et al shows slots extending transverse to the slot between the spaced antenna elements of each element of the array for low-pass filtering purposes. However, the Yngvesson et al reference teaches only Cassegrain or other reflector telescope designs, in which the local oscillator signal is injected through an aperture in the reflector. Such arrangements would involve the same difficulties with respect to uniformity of illumination as in the Gillespie et al reference. Furthermore, such Cassegraintelescope arrangements are not optimal for many desired applications of millimeter wave detection technology; they are bulky, difficult to fabricate and sensitive to physical mishandling. A more compact, more rugged and less complex design is clearly required by the art.